We report the direct evidence for the macromolecular helicity inversion of a helical poly(phenylacetylene) bearing l- or d-alanine pendants with a long alkyl chain in different solvents by atomic force microscopy observations of the diastereomeric helical structures. The diastereomeric helical poly(phenylacetylene)s induced in polar and nonpolar solvents self-assembled into ordered, two-dimensional helix bundles with controlled molecular packing, helical pitch, and handedness on graphite upon exposure of each solvent. The macromolecular helicity deposited on graphite from a polar solvent further inverted to the opposite handedness by exposure to a specific nonpolar solvent, and these changes in the surface chirality based on the inversion of helicity could be visualized by atomic force microscopy with molecular resolution, and the results were quantified by X-ray diffraction of the oriented liquid crystalline, diastereomeric helical polymer films.

Facile access to complex systems is crucial to generate the functional materials of the future. Herein, we report self-organizing surface-initiated polymerization (SOSIP) as a user-friendly method to create ordered as well as oriented functional systems on transparent oxide surfaces. In SOSIP, self-organization of monomers and ring-opening disulfide exchange polymerization are combined to ensure the controlled growth of the polymer from the surface. This approach provides rapid access to thick films with smooth, reactivatable surfaces and long-range order with few defects and high precision, including panchromatic photosystems with oriented four-component redox gradients. The activity of SOSIP architectures is clearly better than that of disordered controls.

We report the unprecedented helix-sense controlled polymerization of enantiomerically pure phenyl isocyanides bearing an l- or d-alanine pendant with a long alkyl chain. The polymerization with an achiral nickel catalyst diastereoselectively proceeds, resulting in either a right- or left-handed helical polymer, whose helix-sense can be controlled by the polymerization solvent and temperature. Both the diastereomeric right- and left-handed helical polymers further self-assemble into lyotropic cholesteric liquid crystals with opposite twist-senses. Consequently, the macromolecular helicity and mesoscopic, supramolecular cholesteric twist can be controlled by the molecular chirality of the pendant of a single enantiomeric phenyl isocyanide through the polymerization under either kinetic or thermodynamic control assisted by hydrogen bonds. High-resolution atomic force microscopy revealed their helical conformations and enabled the determination of the helical sense.

The control and fabrication of ordered 2D arrays of molecules, in particular chiral (macro)molecules on substrates, is among the great challenges in materials science as these systems have attractive applications in chemical sensing, electro-optical devices, enantioselective adsorbents, and catalysis.[1] The concept of supramolecular self-assembly has been widely applied to the epitaxial alignments of chiral molecules on crystalline graphite and metals to create chiral surfaces, and their enantiomorphic 2D arrangements have been visualized by scanning probe microscopy (SPM). [2] However, it is still difficult to exploit biological and synthetic helical polymers [3] for the creation of 2D chiral surfaces by self-assembly because of their unfavorable strong surface interactions, which can compete with intermolecular interactions and change their inherent helical conformations, except for preorganized DNA with specific sequences [4] and nanopatterned collagen formed by dip-pen nanolithography.[5] Herein we show that cholesteric liquid-crystalline helical polyacetylenes self-assemble into ordered, 2D layered crystals on highly oriented pyrolytic graphite (HOPG). Flat monolayers epitaxially form on the basal plane of the HOPG, on which rodlike helical polyacetylenes further self-assemble into 2D helix-bundles with controlled molecular packing, helical pitch, and handedness upon exposure to organic solvents. The helical conformations in 2D crystals are visualized by atomic force microscopy (AFM) with molecular resolution and the results quantified by X-ray diffraction of the oriented liquid-crystalline polymer films. Stereoregular (cis-transoid) poly(phenylacetylene) bearing l-or d-alanine residues with a long alkyl chain as pendants (namely, poly-1 L and poly-1 D, respectively; Figure 1 a) were prepared according to a previously reported method.[6] The circular dichroism (CD) spectra of poly-1 L and poly-1 D in dilute benzene solution showed characteristic Cotton effects in the polymer backbone region (250-450 nm) that are mirror images of each other (Figure 1 b), which indicates that the polymers have a predominantly one-handed helical conformation with exactly the same helical structure except for the sense of their helicity.[6] Among a variety of helical polyacetylenes prepared so far, the polymers have an exceptionally rigid-rod main-chain character; this has allowed the formation of the first polyacetylene-based cholesteric liquid crystals in concentrated solutions.[6] The interplay between the chirality of the pendant groups, the macromolecular helicity, and the chirality at a macroscopic level is of great interest from fundamental and biological viewpoints. To this end, we have investigated the structures of the helical polymers on HOPG with AFM. Figure 2 a shows a typical AFM image of poly-1 L spin cast on HOPG from a dilute benzene solution. The poly-1 L molecules spontaneously self-assembled into a highly ordered monolayer as evidenced by parallel stripes with a periodicity of 4.72 AE 0.21 nm. On this 2D ord...

Rodlike polymers with precisely defined architectures are ideal building blocks for self-assembled structures leading to novel nanometer-scale devices. We found that the living polymerization of a single isocyanide enantiomer bearing an l-alanine pendant with a long n-decyl chain simultaneously produced diastereomeric right- and left-handed helices with different molecular weights and narrow molecular weight distributions. Each single-handed, rodlike helical polymer with a controlled length and handedness isolated by a facile solvent fractionation method with acetone self-assembled to form well-defined two- and three-dimensional smectic ordering on the nanometer scale on a substrate and in a liquid crystalline state as evidenced by direct atomic force microscopic observations and X-ray diffraction measurements, respectively.

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.

Direct observations of the helical structures of artificial helical polymers, such as helical polyacetylenes and polyisocyanides, by atomic force microscopy (AFM) are described in this tutorial review. The two-dimensional helix bundle formation of specific helical polymers on substrates under solvent vapor exposure permits us to determine their helical structures, including their helical pitch and handedness, at a molecular level by AFM in the tapping mode. The direct observation of supramolecular helical structures based on stereoregular poly(methyl methacrylate)s is also described.

Rodlike helical polymers with an excess of one-handedness arising from an optically active component incorporated into the main chain or at the pendants often show chiral liquidcrystalline (LC) phases in concentrated solutions or in a melt.[1] Since the 1980s, such LC helical polymers have been extensively studied with much interest. Typical biological macromolecules, such as DNA, [2] polysaccharides, [3] and polypeptides, [4] which adopt an ordered structure such as a helical structure, with a controlled helix sense stabilized by intra-and/or intermolecular hydrogen-bonding networks, also exhibit chiral LC phases resulting from the rigid-rod characteristics of the polymer main chains. Intramolecular hydrogen bonding has been used to construct synthetic helical polymers, such as polyisocyanopeptides, [5] and amino acid bound polyacetylenes.[6] The former helical polymers showed a clear cholesteric LC phase.Recently, we reported the first helical poly(phenylacetylene)s bearing l-or d-alanine pendants with a long alkyl chain (poly-l-1 and poly-d-1, respectively) that showed cholesteric LC phases in organic solvents owing to their main-chain stiffness assisted by intramolecular hydrogen bonds; their persistence lengths were determined to be approximately 40 nm in chloroform, [7] whereas the previously prepared monosubstituted polyacetylenes appear to be too flexible to exhibit LC phases.[6a-c, 8] We also found inversion of the helicity of poly-l-1 and poly-d-1 in response to the solvent polarity; the Cotton effect signs corresponding to the helix sense of poly-1 in benzene were inverted to the opposite signs in polar solvents such as THF and chloroform.[9] Furthermore, the macromolecular helicity inversion process could be directly visualized by atomic force microscopy (AFM), which revealed their diastereomeric helical conformations and enabled the determination of the helical sense. [9,10] We now show a dramatic change in the main-chain stiffness of poly-l-1 [11] accompanied with inversion of the helical sense of the polymer (Figure 1), resulting from the "on and off" fashion of the intramolecular hydrogen-bonding networks in polar and nonpolar solvents as revealed by the changes in their circular dichroism (CD) and IR spectra, persistence lengths, and rheological properties.Figure 2 a shows the CD spectra of poly-l-1 in polar and nonpolar solvents. Poly-l-1 exhibited split-type intense induced circular dichroisms (ICDs) in the conjugated polyene chromophore region. The ICD patterns measured in nonpolar Figure 1. Illustration of the helix-sense inversion of poly-l-1 regulated by solvents with different polarities, leading to diastereomeric helical poly-l-1s with extremely different main-chain stiffnesses. The helical senses of the diastereomeric poly-l-1s were determined by AFM.